Sanding sealer (and vinyl sealer)

Overview

Bob Flexner's position in Popular Woodworking is that "sanding sealer" is a first coat, sold for speed: it sands to powder faster than the topcoat does, so a big job goes quicker. Conventional sanding sealers contain zinc stearate (a soap) to achieve that. He lists the cost: the coat is less moisture-resistant, softer, cracks more easily and bonds less well than the finish itself. A sealer is a first coat, not a primer.

When to use

• Under nitrocellulose lacquer and alkyd varnish, on big projects where sanding time matters (Nitrocellulose lacquer). That is the only case Flexner finds worthwhile.
• Not under oil-based polyurethane or water-based finishes: they sand and powder easily without it (Oil-based polyurethane, Water-based polyurethane).
• As a pore-filling aid: apply a full coat of the finish first, then one or two coats of sanding sealer, and sand back to the finish (Flexner). For real pore filling see Grain filler (pore filler).
• Not as a stain controller: use Pre-stain conditioner and washcoat.

Materials and tools

A sealer matching the topcoat (see products), a brush or spray gun (Spraying with HVLP), and sandpaper from 220 to 320 grit. Stearated sandpaper is Flexner's alternative to sealer, and so is thinning the first coat of the finish by about half, though he says thinning does not improve bonding.

Steps

1. Sand the wood to the final grit and clean it (Sanding schedule, Dust removal and tack cloths).
2. Apply the sealer without thinning unless the label says otherwise. The Minwax page says to apply unthinned for maximum film build, and sand after 1 hour with 180 grit or finer.
3. Sand it back until it powders and feels smooth.
4. Clean the dust and apply the topcoat.

For plywood, Southwick's pre-finishing schedule uses Zinsser SealCoat (dewaxed shellac, 2 lb cut) by roller or HVLP, dried 15 to 20 minutes, sanded with 320 on a block, under a waterborne floor finish (Finishing sheet goods (plywood, MDF, MDO), Dewaxed shellac).

Vinyl sealer

The sources opened do not define vinyl sealer chemically, so this guide does not either. They do say what it is used for. Flexner (Pinholes) recommends vinyl sealer or shellac for the first thin dusting coats of a sprayed lacquer finish because they better resist being redissolved by the lacquer that follows, which helps bridge pores and avoids pinholes. A catalyzed-finishes article advises doing decorative steps within washcoats of vinyl sealer before conversion varnish, and warns that catalyzed finishes do not bond well to sanding or vinyl sealers that contain stearates. Check the label.

Common problems

• Sealer clogs paper. Flexner says shellac clogs sandpaper a little. Dewaxed SealCoat was created for use under polyurethane.
• Finish peels or cracks over the sealer. Flexner's list of costs for stearated sealers: softer, more prone to cracking, poorer bonding. If it matters, leave the sealer out and thin the first coat, or use dewaxed shellac.
• Water-based sealer claims. The grain-raising claim for water-based sanding sealers, such as Aqua Coat's, was never observed by Flexner. See Raising the grain.

Safety and environment

Solvent-based and shellac sealers are flammable; ventilate and keep ignition sources away. Rags and sanding residue from oil-based products can ignite spontaneously (Oily rag safety). Wear a respirator when sanding the sealer.

Related

• Dewaxed shellac, Shellac, Nitrocellulose lacquer, Oil-based polyurethane
• Grain filler (pore filler), Pre-stain conditioner and washcoat
• Finishing resinous and blotchy woods for shellac as a barrier coat